Yarn faults include thick places, thin places, neps, slubs, fly, knots, snarls, and spinners' doubles. The three factors that determine the nature of yarn faults are frequency of occurrence, type, and size. The major yarn fault grading systems are Zellweger Uster AG's Classimat and Keisokki Kogyo Limited's Classifault. The Classimat system offers 23 classification channels including 16 classes of short thick places, three classes of long thick places, and four classes of long thin places. The Classifault system offers 40 classification channels. Certain yarn faults affect loom efficiency. Several researchers developed indexes and formulae to correlate yarn faults with processing and weaving efficiency. Objectional yarn faults generate as many as 56 percent of total yarn breaks during weaving of polyester/cotton yarns. 15 refs.